it actually makes me laugh how people will answer without reading anything.They are used as a precaution. What if the driver is bad? It is better to find that out while using a test load, rather than the actual diode. The small rectifier diodes are used because they have a similar voltage drop to the laser diode we want to simulate. Since diodes have very little to no resistance, the voltage drop across the load of the driver, is going to directly affect the amount of current that the driver is allowed to provide. I hope this helps.